A vehicle having a large radius of turn is known to have a more limited ability to manoeuvre. This is a particular problem in agricultural vehicles such as tractors where there is often a need to manoeuvre within areas having a restricted space. When using implements such as wide-boom sprayers on fields of crops there is also a desire to minimise the area which is flattened by the tractor whilst still covering the entire field. Having a smaller radius of turn reduces the area flattened when turning on the field headlands for example.
A conventional tractor comprises two axles mounted on a chassis. The front axle comprises steerable wheels mounted at the ends of the axle. As the general trend, farm machinery is progressively increasing in size which, in turn, has lead to an increase in the size of tractors. Larger tractors generally require larger wheels, both larger in diameter and in width. Combined with the constraints on overall tractor width, larger steerable wheels reduce the angle at which the wheels can turn without catching on the engine compartment of the tractor for example. There is, therefore, a desire to find alternatives to the conventional steering arrangement.
UK patent application GB-2,392,657, the content of which is incorporated herein by reference, discloses a compound steering mechanism wherein the axle pivots laterally relative to the chassis. Two plan views of the arrangement from GB-2,392,657 are replicated herein as FIGS. 1 and 2. The same reference numbers will be used for the sake of simplicity.
An axle 20 is connected to an axle support 1 through a link (not shown) which extends upwardly from a ball-and-socket joint on the axle 20 to a ball-and-socket joint on the support 1. The rear part of the axle 20 is connected to the support through a further ball-and-socket joint allowing the axle to pivot in an arc about this joint. With reference to FIG. 1, wheel hubs 23 are steered by means of a steering ram 14 and track rods 15 attached to respective brackets 21. A connecting link 25 is attached to the support 1 forward of the axle, and to a bracket 24 connected to a steerable hub 23.
The connecting link 25 causes the axle 20 to pivot relative to the support 1 in response to operation of the steering ram 14, as shown in FIG. 2, such that the axis Y-Y of the axle moves from being substantially perpendicular to the longitudinal axis to being angularly displaced with respect to the longitudinal axis. By moving the axle relative to the chassis, the turning angle can be increased.